<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
    // 父类有加法方法
    class Father {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }
        sum() {
            console.log(this.x + this.y);
        }
    }
    // 子类继承父类加法方法，同时，扩展减法方法
    class Son extends Father {
        constructor(x, y) {
            // 利用 super() 调用父类的构造函数
            // super 必须在子类 this 之前调用
            super(x, y);
            this.x = x;
            this.y = y;
        }
        subtract() {
            console.log(this.x - this.y);
        }
    }
    var son = new Son(5, 3);
    son.subtract();
    son.sum();
    </script>
</body>
</html>